1. Field of the Invention
The device of the present invention relates to a new and improved assembly for linearly checking or dampening the closing motion of doors of variable weights and to a new and improved method for obtaining linear dampening of the closing motion of such doors.
2. Description of the Prior Art
Presently, at least one of the doors, typically the rear door, of a transit vehicle such as a bus is of the push-open, spring-close type. A passenger desiring to depart from the vehicle may manually push the doors open, and, upon release of the doors after departure, a spring functions to close the doors to their original position. If a passenger departing such a vehicle releases the doors and they close too rapidly, a second passenger preparing to leave the vehicle may be struck and harmed by the closing doors.
The rapid closing of the doors may occur in checking assemblies using compressible fluid, since during the initial closing motion of the doors the piston in the checking assembly is compressing a large volume of the fluid and sufficient pressure is not immediately developed to dampen the motion of the piston and thus the doors until the volume of compressible fluid has been significantly reduced. For this reason, the initial portion of the closing motion of the doors may be more rapid than desired.
It is also possible to open the doors partially and allow closure thereafter such that the volume of fluid compressed by the piston is so small that the fluid is not compressed to the proper amount resulting in insufficient dampening of the motion of the piston and thus the doors.
Both of the above conditions result in nonlinear dampening characteristics of checking assemblies using compressible fluid.
In present day systems there are two fluids that are used in checking assemblies for dampening the closing motion of doors. The first is an incompressible fluid such as in a hydraulic system using oil wherein compressibility of the fluid is not a problem. In these systems, as long as there is no air space within the door check cylinder, there will be substantially linear dampening throughout the closing movement of the door. However, hydraulic systems or systems using incompressible fluid are expensive, normally require complicated valving and require frequent adjustment to compensate for temperature variations. In addition, the possibility of leakage is undesirable in highly traveled public areas.
For these reasons, the second type of system, most normally pneumatic systems, are preferable. However, present pneumatic systems are incapable of complete linear dampening throughout the entire closing motion of a door. Moreover, if a single system is to be offered universally for doors of different weights and linkage, wear over a period of time must be compensated for by the system.
A typical pneumatic system is illustrated in U.S. Pat. No. 3,380,110. The typical linkage and check assembly of the type illustrated in this patent are subject to the limitation that the spring extension forces that tend to close the open door and the frictional forces developed by the door during closing are nearly equal in magnitude near the closed door position. This characteristic is a consequence of the design necessary to package a mechanical linkage in the available space above a typical transit bus door. The equality of frictional and spring forces near the door closed position results in tolerance buildups, linkage wear and other variables that combine to prevent the door from reliably closing. This situation, of course, is undesirable in doors used on public transit vehicles.
Additionally, prior art check assemblies such as that disclosed in U.S. Pat. No. 3,380,110 are open loop systems that use as a source of compressible fluid the atmosphere, thus requiring a filter on the inlet orifice. These assemblies also suffer from wear on the piston in the check assembly due to debris introduced into the system from the atmosphere. Furthermore, linear dampening through the entire closing motion is not achieved in such a system.
A typical system that more nearly obtains linear dampening is illustrated in U.S. Pat. No. 3,010,433. Therein is disclosed a checking device using check valve assemblies at both ends of the checking device housing. One of the assemblies controls the vacuum created behind the piston as the door closes. By regulating the magnitude of this vacuum, the piston movement can be initially checked by reducing the amount of air introduced behind the piston. However, such a system is an open loop system resulting in undesired wear on its components. Moreover, different check valve assemblies must be incorporated in the checking device depending on the weight of the door attached to the checking device. Also, change in frictional resistance to the door movement occurs as the linkage wears requiring periodic changing of the check valves since these valves operate as fixed orifices.
As illustration of a hydraulic system employing a bypass is presented in U.S. Pat. No. 3,722,920. This system includes a variable orifice in the bypass. However, a bypass of this type is not utilized to obtain linear dampening of the movement of the piston, but rather to allow increased flow over a predetermined portion of the piston movement.